The use of bone pins and plates for reducing fractures is well known in orthopedic medicine. The pins and plates extend across discontinuities in a bone to fix the broken ends in relation to each other to reduce pain and promote rapid healing without deformity. These devices are secured to the bone by bone screws or nails driven into the bone. More recently, pins, rods, plates and cages have been used to stabilize bone and joints that have deteriorated naturally or as a result of prior trauma.
The interface between the bone screws and the bone presents problems of stability and long term usage that have been addressed in different ways. One of the major problems is usually termed as back-out. This defines the condition in which the screws attaching the plate to the bone loosen over time, either relative to the bone or the plate or both. Severe back-out results in the bone screw working itself out of the bone and/or plate resulting in instability of the bone or joint. This situation results in increasing pain and danger from the instability, as well as, the movement of the screw. There may be several reasons for the back-out but anatomical stresses from body movements contributes greatly to the problem.
Bone plates are usually attached to adjacent vertebrae to reduce pain due to injury or deterioration of the intermediate disk. The plate spans the intervertebral space to stabilize the vertebrae. Also, bone plates are used to reduce breaks and stabilize bones in other parts of the body. Pedicle screws or bone screws are inserted through apertures in the opposite ends of the plate into the respective vertebrae or on opposite sides of a break. Due to anatomical forces on the skeleton, the screws sometimes back out of the bones and plates.
Prior art devices address the problem of back-out by use of secondary locking screws that hold the bone screws in the plate. The locking device engages the head of the bone screw and is tightened to fix the screw to the plate and, thus, the bone. Such devices are not particularly suited for deployment on the anterior aspect of the spine because of the close proximity of vital soft tissue organs which dictate a smooth, low profile, contoured surface. Michelson, U.S. Pat. No. 6,454,771, discloses a bone plate for anterior cervical fixation. The plate has several holes for receiving bone screws. A locking screw mechanism is used to overlay the screw heads.
An expandable insert for placement between vertebrae is disclosed by Paes et al, U.S. Pat. No. 6,436,142. The device is in the nature of a lag screw and can expand with the insertion of an expansion screw.
U.S. Pat. No. 6,342,055 to Eisermann et al discloses a bone plate with bone screws having a snap-in retainer securing the heads to the plate.
Geisler, U.S. Pat. No. 6,231,610, discloses a bone plate with diverging bone screws and serrations on the plate to increase holding power.
U.S. Pat. No. 6,224,602 to Hayes discloses a bone plate with multiple bone screw holes which may be covered by a sliding locking plate. The bone plate has an undercut channel to hold the locking plate in contact with the screw heads. The locking plate is held to the plate by a locking screw once it is slid to the desired position.
Aust et al, U.S. Pat. No. 5,603,713, discloses an anterior lumbar plate attached by screws with various angular connections to the spine.
Published application, US 2004/0102773 A1, to Morrison et al, uses the ends of the bone plate to cover the heads of the bone screws.
U.S. Pat. No. 6,740,088 B1, to Kozak et al uses extra set screws to interfere with the heads of the bone screws.
U.S. Pat. No. 6,730,127 B2 to Michelson attaches an overlay to the plate to partially cover the heads of the screws.
What is needed in the art is a bone plate with a simple screw lock that does not add extraneous components to the combination.